PAC-1 (pronounced "pack one") was discovered at the University of Illinois at Urbana-Champaign during a process that screened many chemicals for anti-tumor potential. This molecule, when delivered to cancer cells, signals the cells to self-destruct by activating an "executioner" protein, procaspase-3. Then, the activated executioner protein begins a cascade of events that destroys the machinery of the cell.
The problem lies when one part of the apoptosis pathway is broken. Normally, the balance between cell division and apoptosis is rigorously regulated to keep the integrity of organs and tissues. Examples of broken apoptosis pathways occur in many cancers. If old lung cells cannot self-destruct to make room for new lung cells, a large mass of cells form and a tumor is made.
In many cases, the apoptotic pathway is disrupted because procaspase-3, the executioner protein, cannot be activated by the cell. This is analogous to an executioner who does not have orders to kill. Without the orders, the condemned will not die. The same analogy can be made with procaspase-3. Without activated procaspase-3, the apoptotic cascade will not occur and the cell will not destroy itself no matter how necessary it may be. PAC-1 acts a replacement order that works and bypasses the lawyers, court orders, and governor's calls. It will activate procaspase-3 indiscriminately.
In cells, the executioner protein, caspase-3, is stored in its inactive form, procaspase-3. This way, the cell can quickly undergo apoptosis by activating the protein that is already there. This inactive form is called a zymogen. Procaspase-3 has a “safety catch” made of three aspartate amino acids. When this safety catch is released by the cell, procaspase-3 is activated to caspase-3, which starts the apoptotic cycle. PAC-1 cleaves these three amino acids to activate procaspase-3 into caspase-3. Also, caspase-3 further activates other molecules of procaspase-3 in the cell, causing an exponential increase in caspase-3 concentration. PAC-1 facilitates this process and causes the cell to undergo apoptosis quickly.
Unfortunately, a selectivity problem arises because procaspase-3 is present in most cells of the body. However, it has been shown that in many cancers, including certain neuroblastomas, lymphomas, leukemias, melanomas, and liver cancers, procaspase-3 is present in higher concentrations. For instance, lung cancer cells can have over 1000 times more procaspase-3 than normal cells. Therefore, by controlling the dosage, one can achieve selectivity between normal and cancerous cells.
Thus far, PAC-1 seems promising as a new anti-tumor drug. It is synthetically available and a few mouse trials have been performed with moderate success. PAC-1 is the first of many small molecules to directly influence the apoptotic machinery of cells.